Space heaters vs turning up the heat?
November 4, 2013 10:56 PM   Subscribe

I live in a drafty old farmhouse. My roommates and I are currently waging a thermostat war--one says it's cheaper to to have the heat for the whole house at 70 than it is for him to heat his room with a space heater. I maintain that its probably way more expensive to heat the entire drafty house. Who's right? We have electric heat.

Brownie points for citing sources I can use to support my argument (if I'm right).

A few details:
Our house is a small, 2 story, 3 bedroom farmhouse.
His space heater is new but low quality.

I also realize that there is probably a diplomatic way to do this, but that is not the question.
posted by Grandysaur to Home & Garden (26 answers total) 4 users marked this as a favorite
Do you have a way of checking your electrical meter? I think there are so many variables at play here that the only surefire way to test this would be to do each thing for a few days and measure the energy output.
posted by showbiz_liz at 11:05 PM on November 4, 2013 [1 favorite]

If it's all electric heat, then heating one room will always be cheaper than heating the whole house. All electric heaters are 100% efficient, even low quality space heaters.
posted by ryanrs at 11:14 PM on November 4, 2013 [6 favorites]

My electric bill shows me how much I used each day. Does yours do that? That would be one way to find out--use the space heater one day, use the central heat the other, and then check the amount of electricity used each day.
posted by needs more cowbell at 11:36 PM on November 4, 2013

Unless he has a really inefficient space heater then it will be cheaper to use that than to heat the whole house.

Also you should try covering your windows with Scotch Windows insulation kit available on Amazon. Really Cheap and easy to use. You cover your Windows on the inside with the plastic film and it prevents a lot of the draft from coming through the creveses. Works better than regular insulation tape. It really helps to keep the place warmer.
posted by manderin at 11:45 PM on November 4, 2013 [4 favorites]

It's probably cheaper to heat the smaller space instead of the larger small.
posted by Brandon Blatcher at 12:49 AM on November 5, 2013

As ryanrs says, for practical purposes all the electricity going into electric heaters is expressing as heat into the space being heated - where else could it go? Inefficiency is not really an issue. The bigger the space being heated the more energy it is going to take to get to the same temperature.

I assume you're renting so don't want to invest in any significant household improvements but you might consider a hunt for draughts and see what you can do to block them, plus take manderin's suggestion and put coverings on windows. Wear jumpers obviously.
posted by biffa at 12:53 AM on November 5, 2013 [2 favorites]

Best answer: No such thing as an inefficient electric space heater. Every watt it uses goes into heating. Its overall system efficiency, from block of coal to BTU is low compared to say, a kerosene heater, and its heat density is lower (BTUs/hr), but it's efficient as hell. (A BTU is 1 pound of water heated 1 degree F.)

Most space heaters are 1500 watts. There are 3415 BTUs in 1500 watt hours. At 120Volts, that's about 12 amps on a normal 20 amp circuit. If you use two, you'll usually trip a breaker. The main house heat uses 240 volts, and can get the same power from half the current and usually puts out a lot more BTUs.

The load of the room is the volume of air times the density of air, more or less. You heat up the air and the air heats up what's in the room. How much depends on the mass and thermal characteristics of what's in the room. A room full of Cheerios would heat up a lot faster than a room full of lead shot, for instance. Mostly, YOU perceive air temperature.

Because when you heat a house, you heat all the house elements, it's arguably less efficient to heat it than the room, but maybe a lot faster. If you don't occupy the room a lot, it's going to be cheaper to heat it locally (space heater) than globally (full house heat.) A room is usually interfaced to the outside world with windows, which represent an energy leak. If your room has windows, you have to deal with the heat loss from them and/or any heat loss from air infiltration from a colder source. A room with a lot of windows and/or leaky doors is harder to heat than one with none, like an interior room.

Not sure if that's what YOU want to hear, but that's the way it works. Kinda. (Lots of omissions.) One thing in favor of centralized heating is that it's not prone to burning down the building because of misuse. Any idiot can catch a blanket on fire with a space heater. Happens in my town literally every year. Once, it burned down my office. Ouch.

You need to understand the model to knowledgeably argue your point, first. Diplomacy is secondary to technical comprehension.
posted by FauxScot at 1:04 AM on November 5, 2013 [13 favorites]

Zone heating (ie heating a smaller enclosed space) is always more efficient than heating an entire house. Those crappy adverts talking about Amish built heaters in Parade magazine that talks about saving money on your heating bills is only because they allow for zone heating. They allow you to heat your living room at the expense of the rest of the house thereby using less energy and saving money.

Think of it like heating lakes in the summer. Each lake receives the heat in the form of sunlight. Smaller ponds heat up faster than larger lakes, even though the larger lake can absorb more sunlight. This is because it takes much more energy to heat the larger deeper lake than s shallow pond that the difference in energy input is irrelevant.
posted by koolkat at 1:12 AM on November 5, 2013

His space heater is new but low quality.

The solution here is to get him, or have him get a quality space heater, one like this(or, with a much less accurate thermostat this much cheaper version which heats just as well). The issue here, which has caused many a roommate battle in past share houses in my life is that central heat makes everywhere warm, whereas with shitty space heaters there's often an area close to them that's warm. A proper heater that isn't just the cheapest possible heating element in front of a wimpy fan. It's also literally impossible to set something on fire with one of those(i've tried, several times to prove that point. it just tripped the thermal breaker for a couple minutes both times)

I was a huge proponent of running the main crappy electric radiators which had to run for hours to make it warm in my part of the house until i got one of those. As FauxScot correctly stated, what matter is the air temperature you perceive. Those move a lot of air, and heat it all up very quickly. The thing never needed to run for more than a few minutes for the air in my room to feel warm enough that i could pass out without thinking i was going to turn in to an ice block. And that's really what matters, the "seat of your pants" feel of the whole thing.

Fuck, go buy one at somewhere like walmart or costco where they have a pretty no questions asked return policy if you have the box and receipt. Try the thing out for a week and see if it's a workable solution over just running the central heat. Because in my old crappy drafty poorly insulated house running a couple space heaters in 2 or 3 of the rooms was INCREDIBLY cheaper than running the baseboards all the time. Like a not even funny amount cheaper. the power bills when we killed all the breakers to the baseboards were somewhere between 1/3rd and half what they were before because we were sealing up the rooms we wanted to keep warm with closed doors and hanging blankets on doorless doorways and really trying to trap the heat from the few space heaters in those specific rooms. Less air to heat = less energy to use, and less time to heat that air and space. The lake analogy above is spot on.
posted by emptythought at 1:58 AM on November 5, 2013 [2 favorites]

Ducted central heating and space heaters both warm the air, some of which then warms you. But most of the air warmth gets wasted on heating walls with no feelings and escaping through vents and gaps to the outside.

Radiant heat warms whatever the glow lights up. Small personal radiators are usually directional, so you can easily arrange for most of what the glow lights up to be you, which avoids paying to heat stuff that isn't you. Getting warm from a radiator typically costs about half as much as getting equivalently warm from a space heater.

A small personal radiator will also usually start making you feel warm more quickly than a space heater, simply because it's heating you more directly.

Fixed radiant panels with room thermostats work more like space heaters than personal radiators. They're not particularly directional, they usually run at much lower temperatures than a personal radiator, and work mostly by convective heating of room air. Most of the warm airflow from a fixed radiant panel runs directly upward from the panel and ends up making a pool of very warm air just under the ceiling. They're quiet and unobtrusive and a very low fire risk but they're horribly wasteful. If that's the kind of central heating your drafty farmhouse has it will cost you a fortune to keep the house at 70°F while it's cold outside.

On the other hand: if you have ducted central air, and that air is heated by a reverse-cycle heat pump rather than by direct electric heating, and it's not cold enough outside to stop the heat pump working efficiently, and all three bedrooms are occupied by people who would otherwise be running personal space heaters in them rather than personal radiators, and the ducts can be blocked off in rooms you don't want to pay to heat that also don't have thermostats in them, then and only then might your housemate have a point. A properly working heat pump typically consumes about 1/3 of the power of direct electric heating to maintain a given space at a given temperature, and that fact might be enough to offset the waste involved in heating some of the spaces none of you are in.

If that's what you have, do what showbiz_liz suggested: run each method for a week and take before-and-after electricity meter readings. If not, then your housemate is just flat wrong, and if he wants to press the point and run his own test, make sure you bet money on the outcome before agreeing to it (and make sure he's not rigging it by running his space heater flat-out while nobody is home).
posted by flabdablet at 3:34 AM on November 5, 2013 [1 favorite]

totally with flabdablet... no substitute for testing. all you need is a clock, a notepad/pen, a thermometer and a method.

compare approaches.

temperature CHANGE over time is the relevant quantity. how long to make a 20 degree change in the same place in his room with space heat versus whole house heat? that way, you can avoid time considerations inherent in more general questions... like "How long to get the house to 70?" Depends on when you do it. Looking at change minimizes the impact of that factor.

A quick way to check the power levels used in whole house heating.... go find the breaker panel for the house. Find the breaker(s) for heating. Take 80% of whatever that value is (i.e., if they are 50A breakers, use 40A). 40A x 240V = about 10 KW. Assume the space heater is 1.5 KW. Where I live, a KW hour costs about $0.10-12. So house might cost $1/hr to heat up, and room would cost $0.15/hr. You can also proportionally allocate the heating burden based on the square feet of the room versus total house.

Many ways to assess this. Data is good. Get some.
posted by FauxScot at 4:10 AM on November 5, 2013

Since all electric resistance heaters are equally efficient, it's certainly cheaper to heat one room using resistance heat than it is to heat the whole place with resistance heat. But we can't say whether or not heating the whole house is "way more expensive" because we don't know what the rest of the occupants would be doing for heat if the housemate in question used his space heater. Your profile puts you in Montana, where you're all going to need heat from one source or another. If it's a case of one person preferring their room warmer than the others, then the amount saved by boosting that room's temperature with a space heater instead of using the central heating system might be rather small.
posted by jon1270 at 4:22 AM on November 5, 2013

How about compromising at a lower temp? If you are in MT, heating a house to 70 degrees is probably costly since it is way above your outside temp at a time when utility prices tend to go up. How about setting the thermometer at 67 or 68 (looking at your kw/hr $)? I'm thinking that winter heat (which tends to dry out the air on top of the already dry winter air) shouldn't be something people are walking around in their t-shirts. But maybe I'm jaded, we set our thermometer at 60 at night and 64 during the day.
posted by lasamana at 4:39 AM on November 5, 2013 [1 favorite]

You're almost certainly right, but I agree with emptythought's implication that this isn't really about that -- your roommate just prefers a house that's warm* everywhere to one where only his room is warm. So I'm not sure that figuring out the truth here will be all that helpful about resolving the underlying difference.

*Unless he has some sort of real honest to god medical condition, 70 is completely bonkers.
posted by ROU_Xenophobe at 5:06 AM on November 5, 2013 [2 favorites]

Either way, it is worth looking at easy ways to reduce the draftiness of that farmhouse. When I lived in drafty rental housing, every fall we broke out the Mortite rope caulk & went over all the windows - it is cheap, reversible, and surprisingly effective.
posted by mr vino at 5:17 AM on November 5, 2013

"No such thing as an inefficient electric space heater. Every watt it uses goes into heating."

My friend had a very very old space heater. He claimed it was almost 30 years old and it sure as hell looked like it was. IT was HUGE and the amount of heat that came from it didn't seem to justify it's size. I'm not an expert, but that thing seemed to be pretty inefficient. Some people will hold on to appliances long past their expiration date.
posted by manderin at 5:24 AM on November 5, 2013 [2 favorites]

@manderin... engineers use 'efficiency' in a technical sense, not the colloquial sense. Colloquial use is usually for what most people would call 'effective', which is different than efficient.

Heat out (i.e., power) / power in = efficiency. All electric heaters are 100% efficient. Where else would the heat go? Ideally, all of it would heat YOU up, but some of it heats the space heater, some the air, some the furniture close by. Every watt is accounted for. that's a different story and subjective, depending on the end use.
posted by FauxScot at 5:59 AM on November 5, 2013 [1 favorite]

I'm not an expert, but that thing seemed to be pretty inefficient.

This is a natural thing to intuit, and completely wrong.
posted by jon1270 at 6:03 AM on November 5, 2013 [3 favorites]

All electric heaters are 100% efficient

That really depends on whether you're comparing the electric power consumed by the heater to the total heat power coming out of it, or to the total heat power delivered to the user i.e. the person who wants to be warm.

Of course it's arguable that total heat power coming out of the heater is the only fair thing to rate the heater's efficiency on, and that measuring the total heat power delivered to the user is more a measure of the efficiency of the user's total heating environment.

But it's also arguable, I think quite fairly, that if the design of the heater itself makes it direct most of its heat output into the user and minimize the amount dumped uselessly where it isn't needed - as a radiator can compared to an air heater, or an electric blanket can compared to a radiator - then the less wasteful design is the more efficient. This is end-use efficiency, and it's an important measure to think about.

Effectiveness is also not necessarily subjective: you could measure temperature rise in an instrumented dummy to gauge it. But efficiency and effectiveness are certainly two different things. The most wasteful, least end-use efficient whole-house central heating system it's possible to design might well end up being tremendously effective if you let it suck up enough electric.

With all that in mind, it's still not fair to conclude that an ancient space heater that doesn't get you particularly warm is inefficient compared to a newer space heater. It may just be that its heater element is old and corroded and doesn't consume as much electric power as it did when it was new; so even though it's not getting you as warm as a newer unit might, it's also costing you less to run.
posted by flabdablet at 6:28 AM on November 5, 2013 [4 favorites]

If you go the space heater route, make sure the central heating thermostat is still being satisfied so your pipes don't freeze up!
posted by leahwrenn at 7:45 AM on November 5, 2013

total heat power delivered to the user i.e. the person who wants to be warm.

If you want to maximize heat delivered to the person, buy a set of electric jacket and pants. But that's almost certainly overkill for indoor use unless it's 30 F inside your house.
posted by ryanrs at 8:44 AM on November 5, 2013

Regarding the space heater being of low quality, I'd like to emphasize what has been said by quite a few others, and add an explanation about "efficiency".

Efficiency is basically a measure of the proportion of energy that is used to perform work; the rest of the energy is wasted. So, if some apparatus has a 30% efficiency rating, it means that 30% of the energy do "work" and the rest is "wasted". By "work", we usually refer to making something move (say a motor, a compressor, etc.). Or, say for lightbulbs, the "useful" energy is in the form of light. What do we mean by energy being wasted? We mean that it is discarded in the form of heat. (For incandescent lightbulb, 95% of the energy or so is wasted as heat.)

An electric heater does not perform any useful work; it only wastes energy ... in the form of heat.
posted by aroberge at 9:56 AM on November 5, 2013 [1 favorite]

Your room mate should get some fleece overgarments because heating a drafty old house to 70 degrees in a cold climate is nuts. They can make up the difference with a space heater.
posted by Good Brain at 10:27 AM on November 5, 2013

... one says it's cheaper to to have the heat for the whole house at 70 than it is for him to heat his room with a space heater. I maintain that its probably way more expensive to heat the entire drafty house.

People tend to get all hung up on figuring out the amount of heat going in to heat the house. A clearer way to think about it is to consider that you are paying for the heat that escapes from the house. So when comparing two alternatives, just think about which loses heat faster because it is only the heat that escapes that needs to be replaced. Think about the flow of heat through the exterior walls out of the house. The greater the temperature difference between inside and outside, the faster the heat flows. Think of heat flow like air flow. The higher the temperature/pressure, the faster the leak.

In your situation it should be obvious that heating the whole house causes the house to lose heat faster, because you have more surface area exposed to the higher heat so it flows out more quickly. If you have only one hot room, just that small exterior surface area loses heat quickly. The rest of the house loses heat more slowly because it is cooler. So think about heat going out, not the heat going in.
posted by JackFlash at 10:44 PM on November 5, 2013 [2 favorites]

You wouldn't believe the difference made by plastic over every window (it doesn't have to look ugly; use the good stuff, shrink it with heat, and trim the excess) and those fabric things that slide under drafty doors (the ones that have a ridge in the middle and stay on when you open and close the door). If you cut down the leaks he may even find that a space heater in one room is enough.
posted by Pomo at 8:04 AM on November 6, 2013

Not all electric heaters are 100% efficient. Heat pumps (AC units that can run backwards) can greatly exceed 100% (they bring more heat energy into the home than they consume, because they're not converting the electricity to heat, they're using it to gather energy from outside)

If the central heat is an electric heatpump, then the loses from heating a larger area are probably close enough to offset by the higher heating efficiency and lower household interpersonal friction when neither of you are chilly :-)

If both sources of heat are resistive, then you are right - both are of exactly equal efficiency, so heating a subset of the house requires a subset of the energy.
posted by anonymisc at 12:22 AM on November 7, 2013

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